1. Field of the Invention
The present invention relates to a semiconductor device.
2. Description of the Related Art
The present invention is directed to a current fuse element that is a semiconductor device formed on a semiconductor substrate together with a semiconductor integrated circuit, as a storage element used in a one-time programmable (OTP) memory which is programmable only once.
In a semiconductor integrated circuit in recent years, the OTP memory has been an essential element to store redundancy replacement information as to which one of cells of a memory having redundancy, such as a dynamic random access memory (DRAM) and a static random access memory (SRAM), is to be replaced by a redundancy cell, identification (ID) information specific to a chip, or tuning information of an analog circuit.
An electrically programmable fuse (hereinafter, “eFuse”) element has been used mainly as a storage element used in the OTP memory. Types of eFuse elements include a gate-oxide-film breakdown-fuse element, a current fuse element or the like, and various proposals concerning eFuse have been made.
The gate-oxide-film breakdown-fuse element is an antifuse-type fuse element that reduces the resistance by causing a gate oxide film of a metal-oxide semiconductor (MOS) transistor to generate a dielectric breakdown by applying a high voltage to this gate oxide film. On the other hand, the current fuse element is a type of fuse element that changes the resistance value by fusing a wiring or changing a wiring structure by passing a large current to the wiring itself.
A current fuse element as a subject of the present invention is explained below. A fuse element using a polysilicon wiring obtained by silicificating a band region of a predetermined width of a surface of a polysilicon layer by reacting this region with a metal such as cobalt (Co), nickel (Ni), and titanium (Ti) is known as a kind of a current fuse element used for an eFuse element. Silicified polysilicon is used as a gate material.
As a method of programming such a polysilicon fuse element, the following method has been proposed. A large current is passed to silicified polysilicon, a metal element of a silicide layer is shifted to the same direction as that of a flow of electrons based on an electromigration phenomenon, and the silicide layer is disconnected, thereby increasing its resistance (for example, see “Electrically Programmable Fuse (eFUSE) Using Electromigration in Silicides” IEEE Electron Device Letters, Vol. 23, No. 9, September 2002).
A fuse element using this polysilicon wiring structure does not require a high voltage at a programming time as compared with a gate-oxide-film breakdown-fuse element. Therefore, this fuse element can be programmed without giving excessive stress to transistors constituting a logic circuit.
However, the fuse element using the polysilicon wiring structure cannot have a large resistance value after the programming because of fluctuation in a program voltage, when a metal element does not shift sufficiently based on the electromigration phenomenon. Therefore, there is a risk that the resistance value varies after the programming and that a sufficient current ratio before and after programming the fuse element cannot be obtained.
Furthermore, when there is a region in which a silicide layer is not uniformly generated in the fuse element, the initial resistance value may be varied and the fuse element may lose its reliability.